In recent years, with the drastic increase in the communication traffic due to the prevailing of Internet, the communication is now being replaced to optical communication by which data transmission in with large capacity can be attained at a high speed from the conventional communication using metal wiring with which the information amount in communication is limited. Optical communication has advantages over the conventional communication through electric wiring in that broadband is available and that it is not influenced by noise. As examples of optical communication, Japanese communication makers are now constructing FTTH (fiber to the home) network in which quartz-based optical fibers reach homes, and European automobile companies are now developing MOSTCO (The Media Oriented Systems Transport Corporation) which is a standard of on-vehicle LAN using plastic optical fibers. In addition to this long distance to middle distance communications, for the middle to short distance communications, the inter-apparatus and intra-apparatus interconnections between domestic computers, electronic switching systems and the like are also now replaced to optical interconnections using optical fibers or sheet-like optical waveguides from the present communications using wire harness. Above all, as a material for the optical interconnection between boards, in a board, between chips and in a chip, flexible polymer optical waveguides made into a resin film are now gathering hope.
In such a trend to proceed optical wirings, polymer optical waveguides and plastic optical fiber sheets which are inexpensive and have good ease of handling, not the quartz-based single mode optical fibers which are expensive and have poor ease of handling, are drawing attention. For example, a method for producing an optical waveguide sheet by compression molding of a laminate in which a core sheet is sandwiched between a pair of cladding sheets has been proposed (JP 2001-281484 A). However, by such a method, the core layer and the cladding layers deform to constitute waveguides, and since stress or orientation is suffered among the molecules at the deformed site and vicinity thereof, anisotropy of refractive index is generated, so that dispersion of the propagating light is large, and information transmission with a high density is likely to be difficult, which is problematic. Further, since the core layer is continuous in directions other than the direction of travel of light, light is likely to leak.
A method for obtaining an flexible embedded type optical waveguide has also been proposed wherein on a Cu—Si substrate, 1) a under-cladding layer and a core layer are formed successively; 2) cores serving as waveguides are formed by photolithography and dry etching; 3) the resultant is covered with a upper-cladding layer; and 4) the substrate is peeled off (JP 08-304650 A). In this case, since the optical loss is reduced to some degree when compared with the former, since a vacuum process, spin coat, photolithography, dry etching and the like are necessary, only a batch process can be employed, so that the cost is high, which is problematic. Moreover, it is difficult to obtain a film having a long size or a large area. Further, it is difficult to form cores whose cross-sectional shape is one other than rectangular, the optical loss is high when compared with the circular or oval cores. Still further, since the core layer is formed by the spin coat method, there are cases where the optical loss due to the roughness of the interface is in an unacceptable level. In addition thereto, a method wherein selective polymerization, reactive ion etching (RIE) and photolithography are combined (JP 2004-206016 A), direct exposure method (JP 2003-185860 A), a method based on injection molding (JP 2003-172841 A), and photo-bleaching method (JP 2004-012635 A) and the like are known. However, any of these methods has a problem in that the number of steps is too many, so that the production time is long, production cost is high, and production yield is low. Further, since the cores and cladding constituting the polymer optical waveguides are formed by utilizing a reaction phenomenon of photo or thermal hardening resin, it is also difficult to produce a film having a large surface area and long size.
On the other hand, as a production method of a plastic optical fiber sheet (also called fiber ribbon), a method is known wherein a plurality of optical fibers are passed through guide rings arrayed at prescribed intervals and unifying the resultant with an adhesive (JP 60-178405 A). As a method for producing sheet-like lightguides in which the peripheries of the composites serving as lightguides are fused and covered with a resin or the like, a method of using a die has also been proposed wherein each of the fiber arrays is molded in a batch (JP 04-043304 A). However, by the former production process of optical fiber sheet, it is difficult to accurately position and array the optical fibers, so that the production yield and high cost are problematic. On the other hand, by the latter production process, there is a problem in that it is difficult to array the cores keeping the shape of all of the cores in the transverse direction of the film, and the variation in the shape of the obtained core is large. There is also a problem in that since a fluorine-containing resin is used in a large amount for forming the cladding walls connecting between core and adjacent core, the cost is high, and the low self-supporting property of the fluorine-containing resin itself is also problematic. That is, there is a problem in that since it is difficult to keep the state wherein the cores are arrayed linearly and uniformly in the transverse direction of the film (since it is difficult to adjust the position of the cores), optical interconnection is difficult.
On the other hand, the present inventors have proposed an optical waveguide film in which a plurality of cores is arrayed by using a melt extrusion process (JP 2004-205834 A). By this method, although an optical waveguide film with which the loss is low and which has a large area and long size is easily obtained, there is a task to reduce the variation in the optical waveguide performances among the cores.